When the lights came up at intermission at “The Gershwins’ Porgy and Bess” Saturday night, audience members were surprised to see Michelle Obama had made a stealth appearance. “She came in quietly after the lights went down,” said a spy at the Richard Rodgers Theatre, “so almost no one in the audience realized she was there.” They added that the first lady’s arrival was “a total surprise. Only producers Jeffrey Richards and Rebecca Gold knew she was going to be there in advance.” The cast of the Tony-winning show, including Audra McDonald, was clued in at intermission, and Obama then went backstage to take pictures with them. She was flanked by Secret Service and her sister-in-law, Maya Soetoro-Ng.